1. Field of the Invention
The present invention relates to a small-sized stepping motor adapted for office automation instruments.
2. Description of the Prior Art
As shown in FIG. 5, a conventional permanent-magnet (PM) type two-phase stepping motor is constructed of: a pair of can-type stator yokes 11, 11; a pair of flat-type stator yokes 12, 12; a pair of coils 13, 13; and, a rotor 4.
The conventional stator yokes are produced under a press working process of a material or sheet metal. In case of the flat-type stator yoke, as shown in FIG. 4, the stator yoke is produced through sequentially: a disk-blanking step (a) in which a disk blank is punched out of a sheet metal; a deep-drawing step (b) of the disk blank; a tooth-blanking step (c) of the thus deep-drawn disk blank; and a tooth-bending step (d) of the thus bent tooth of the deep-drawn disk blank. In production of the can-type stator yoke, similar sequential four steps may be used.
Problems to be solved by the present invention will be now described.
In a conventional method for producing the stator yoke of the small-sized stepping motor under the press working process, since the toothed portion of the stator yoke has a small size, press-working tools for producing such toothed portion are also small in size. Consequently, such press-working tools tend to break when they are subjected to a mechanical impact produced in the blanking operation. The thus damaged tools impair the product's or stator yoke's quality and result in poor product yields.
In addition, in the press working process, the material for the stator yoke must be a thin sheet metal subjected to severe thickness restrictions. Further, in a small-sized stepping motor, the stator yokes including toothed portions are subjected to severe shape restrictions.
Furthermore, in the press working process, it is necessary for the conventional stepping motor to produce two pieces of the stator yoke regardless of its type (i.e., regardless of the can-type or the flat-type), as shown in FIG. 5 with the reference numerals 12, 13.
In addition, the conventional stepping motor (the stator yokes of which are produced under the press working process and are low in specific electrical resistance) has a large in current loss due to such low specific electrical resistance, and, therefore due to the resultant eddy current produced in the stator yoke under the influence of a magnetic flux for driving the motor.
Particularly, the greater speed the motor obtains, the more energy loss the motor produces due to the eddy current. As a result, the percentage of input electric power effectively converted into a form of kinetic energy of the motor decreases, so that the conventional stepping motor suffers from poor torque at high speed.
In addition, since such eddy current is eventually converted into heat, the conventional small-sized motor (which is poor in available heat-radiation area) suffers from a high temperature in operation. Such high temperature adversely affects the conventional stepping motor in reliability.
Consequently, in the conventional small-sized stepping motor, in order to solve this poor-reliability problem due to the high temperature, the motor has its coil coated with an insulating film excellent in heat resistance. However, such insulating film is expensive, and, therefore increases the manufacturing cost of the product.
As described above, the conventional stepping motor (the stator yoke of which is constructed of the sheet metal) is disadvantageous in that: the specific electrical resistance of the stator yoke is low, which prevents the motor from accomplishing its high operation speed and its down sizing.